It has previously been proposed to switch an optically responsive medium resonantly using pulses in antiphase. For example, in EP-A-0 744 648, optically generated charge carriers are produced in an optical medium, in response to a first pulse of optical radiation and are thereafter de-excited by directing a second pulse into the medium. A second pulse occurs whilst the excitation produced by the first pulse remains coherent, and the relative phase of the first and second pulses is selected so that the second pulse destructively interferes with the excitation produced by the first pulse. The apparatus may be used as an optical switch in a modulator or a photodetector.
Reference is also directed to Kobayashi T. et al, “Coherent push-pull transition for ultrafast optical switching”, Quantum Electronics and Laser Science Conference (QELS) 1991-QELS, 91 Technical Digest, paper QWD 21, pages 144-145.
A problem with the prior techniques is that the excitation produced by the first pulse has a coherence time of the order of picoseconds or shorter at room temperature, with the result that the second pulse needs to occur within a very short time after the first pulse in order to achieve coherent de-excitation. As a result, the prior systems have been operated at low temperature in order to extend the coherence time of the excitation produced by the first pulse.